A conventional cardiac pacemaker is an implantable battery-powered electronic device that responds to sensed cardiac events and elapsed time intervals by changing its functional states so as to properly interpret sensed data and deliver pacing pulses to the heart at appropriate times. The pacing pulses are delivered through a lead made up of an electrode(s) on a catheter or wire that connects the pacemaker to the heart. Additional sensing of physiological data allows some pacemakers to change the rate at which they pace the heart in accordance with some parameter correlated to metabolic demand. Such pacemakers, which are the primary subject of the present invention, are called rate-adaptive pacemakers.
The most common condition for which pacemakers are used is the treatment of bradycardia. Atrio-ventricular conduction defects (i.e., AV block) that are fixed or intermittent and sick sinus syndrome represent the most common indications for permanent ventricular pacing. Pacemakers may also be employed to deliver cardiac resynchronization therapy (CRT) to patients having ventricular conduction deficits in which paces are delivered one or more ventricular sites in order to cause a more coordinated contraction. In chronotropically competent patients in need of ventricular pacing, atrial tracking modes such as DDD or VDD are desirable because they allow the pacing to track the physiologically normal atrial rhythm, which causes cardiac output to be responsive to the metabolic needs of the body. Atrial tracking modes are contraindicated, however, in patients prone to atrial fibrillation or flutter or in whom a reliable atrial sense cannot be obtained. In pacemaker patients who are chronotropically incompetent (e.g., sinus node dysfunction) or in whom atrial-triggered modes such as DDD and VDD are contraindicated, the heart rate is determined solely by the pacemaker in the absence of intrinsic cardiac activity. That heart rate is determined by the programmed escape intervals of the pacemaker and is referred to as the lower rate limit or LRL.
Pacing the heart at a fixed rate as determined by the LRL setting of the pacemaker, however, does not allow the heart rate to increase with increased metabolic demand. Cardiac output is determined by two factors, the stroke volume and heart rate, with the latter being the primary determinant. Although stroke volume can be increased during exercise, the resulting increase in cardiac output is usually not sufficient to meet the body's metabolic needs unless the heart rate is also increased. If the heart is paced at a constant rate, as for example by a VVI pacemaker, severe limitations are imposed upon the patient with respect to lifestyle and activities. It is to overcome these limitations and improve the quality of life of such patients that rate-adaptive pacemakers have been developed. Rate-adaptive pacemakers operate so as to vary the lowest rate at which the heart is allowed to beat in accordance with one or more physiological parameters related to metabolic demand.
One way to control the rate of a pacemaker is to measure the metabolic rate of the body and vary the pacing rate in accordance with the measurement. Metabolic rate can effectively be directly measured by, for example, sensing blood pH or blood oxygen saturation. Practical problems with implementing pacemakers controlled by such direct measurements, however, have led to the development of pacemakers that are rate-controlled in accordance with physiological variables that are indirectly reflective of the body's metabolic rate such as body temperature, ventilation rate, or minute ventilation. Minute ventilation varies almost linearly with aerobic oxygen consumption during exercise up to the anaerobic threshold and is the physiological variable that is most commonly used in rate-adaptive pacemakers to reflect the exertion level of the patient.
An even more indirect indication of metabolic rate is provided by the measurement of body activity or motion using an accelerometer. Body activity is correlated with metabolic demand because such activity requires energy expenditure and hence oxygen consumption.
In such rate-adaptive pacemakers that vary the pacing rate in accordance with a measured exertion level, the control system is generally implemented as an algorithm that maps a particular exertion level to one particular target heart rate, referred to as the sensor-indicated rate (SIR). The mapping is accomplished by a rate-response curve which is typically a linear function (i.e., a straight line), but could also be some non-linear function as well such as a dual-slope curve or exponential curve. The responsiveness of the control system, defined as how the target heart rate changes with a given change in exertion level, depends upon the slope of the rate-response curve (or slopes in the case of a dual-slope curve), referred to as the response factor. If the response factor is incorrectly defined, the pacemaker's responsiveness will not be set to an appropriate level. An under-responsive pacemaker will unnecessarily limit exercise duration and intensity in the patient because the heart rate will not increase enough to match metabolic demand, while an over-responsive pacemaker can lead to palpitations and patient discomfort.
The usual methods for setting the response factor of rate-adaptive pacemaker involve exercise testing or continuous monitoring to determine the patient's maximum exertion level to which is associated an individually selected maximum pacing rate. Exercise testing may not always be practical, however, and a patient's maximum exercise capacity can change over time due to, e.g., physical conditioning or illness which increases the need for follow-up visits. Algorithms have therefore been developed that attempt to adjust the responsiveness of rate-adaptive pacemakers automatically in accordance with exertion level measurements made as the patient goes about ordinary activity. Determining a patient's maximum exercise capacity from periodic exertion level measurements, however, is problematical since it is not known how close to the true maximum a periodic maximum exertion level is.